Ray tracing systems can simulate the manner in which rays (e.g. rays of light) interact with a scene. For example, ray tracing techniques can be used in graphics rendering systems which are configured to produce images from 3-D scene descriptions. The images can be photorealistic, or achieve other objectives. For example, animated movies can be produced using 3-D rendering techniques. The description of a 3D scene typically comprises data defining geometry in the scene. This geometry data is typically defined in terms of primitives, which are often triangular primitives, but can sometimes be other shapes such as other polygons, lines or points.
Ray tracing mimics the natural interaction of light with objects in a scene, and sophisticated rendering features can naturally arise from ray tracing a 3-D scene. Ray tracing can be parallelized relatively easily on a pixel by pixel level because pixels generally are independent of each other. However, it is difficult to pipeline the processing involved in ray tracing because of the distributed and disparate positions and directions of travel of the rays in the 3-D scene, in situations such as ambient occlusion, reflections, caustics, and so on. Ray tracing allows for realistic images to be rendered but often requires high levels of processing power and large working memories, such that ray tracing can be difficult to implement for rendering images in real-time (e.g. for use with gaming applications), particularly on devices which may have tight constraints on silicon area, cost and power consumption, such as on mobile devices (e.g. smart phones, tablets, laptops, etc.).
At a very broad level, ray tracing involves: (i) identifying intersections between rays and geometry (e.g. primitives) in the scene, and (ii) performing some processing (e.g. by executing a shader program) in response to identifying an intersection to determine how the intersection contributes to the image being rendered. The execution of a shader program may cause further rays to be emitted into the scene. These further rays may be referred to as “secondary rays”.
A lot of processing is involved in identifying intersections between rays and geometry in the scene. In a very naïve approach, every ray could be tested against every primitive in a scene and then when all of the intersection hits have been determined, the closest of the intersections could be identified. This approach is not feasible to implement for scenes which may have millions or billions of primitives, where the number of rays to be processed may also be millions. So, ray tracing systems typically use an acceleration structure which characterises the geometry in the scene in a manner which can reduce the work needed for intersection testing. However, even with current state of the art acceleration structures it is difficult to perform intersection testing at a rate that is suitable for rendering images in real-time (e.g. for use with gaming applications), particularly on devices which have tight constraints on silicon area, cost and power consumption, such as on mobile devices (e.g. smart phones, tablets, laptops, etc.).